COLUMBUS, Ohio — Mitt Romney and President Obama dashed through several tossup states Monday, urgently conveying to supporters and undecided voters that their single ballot could determine who will be the next president.

Now — after $2 billion in spending, ever-cycling television ads, and stump speeches delivered until the candidates’ voices turned hoarse — the country heads into an Election Day in which the outcome remains too close to call.

“We’ve made real progress, Ohio, but the reason why we’re here is we’ve got more work to do,” Obama said at an arena of 15,500 here, after musicians Jay-Z and Bruce Springsteen pumped up the crowd with a mixture of soulful harmonica and pulsating bass. “Our work is not yet done!”

About three hours later and 7 miles away, Romney’s campaign plane pulled into a hangar at Port Columbus International Airport as trumpets played “Fanfare for the Common Man.” “This is an exciting time — and your voices are being heard loud and clear all around the nation,” he told a rally at the airport following a concert by the Marshall Tucker Band, a country-rock stalwart.

Both campaigns see Ohio as critical to their efforts. Since the start of the year, the candidates, their running mates, and their spouses have combined to visit the Buckeye State more than 100 times. About $120 million has been spent on ads here by the campaigns and groups supporting them.

In a strategic surprise, Romney’s advisers revealed he would extend his campaigning with Election Day rallies in Cleveland and Pittsburgh after voting at his local precinct in Belmont, Mass. His team had previously signaled he would end campaigning on Monday night in New Hampshire.

Obama plans to spend the day in Chicago, with an itinerary that includes a series of interviews with swing-state television stations, then playing a pick-up basketball game, an activity he considers good luck.

For Romney, Tuesday marks the end of a bid that began in the twilight of his term as governor of Massachusetts. If he were to win, he would become the first president of the Mormon faith and would fulfill the dream of his late father, George, who ran for president in 1968.

Romney would almost immediately activate a transition team that has been working quietly in Washington for months and begin the task of appointing Cabinet members, crafting a budget, and trying to figure out how to lead a divided Congress.

“We have to be a united nation — out of many, one,” Romney said at an afternoon rally in Lynchburg, Va., using a Teleprompter to prevent any last-minute hiccups. “By the way, we’re only one day away from a fresh start. One day away from a new beginning.”

“Walk with me, walk together,” he urged. “Tomorrow we’re going to begin a new tomorrow.”

If Obama wins, he will attempt to claim a mandate and turn to the knotty work of finding a way around the so-called fiscal cliff, with budget cuts and expiring tax breaks scheduled to take place at the end of the year unless he can forge a compromise with a lame-duck Congress.

With their decision, voters will signal whether Obama’s signature accomplishment — his health care overhaul — will survive, whether the financial
overhaul will be kept intact, and whether all tax cuts will be extended (as Romney wants) or whether those for the wealthy will expire (as Obama wants).

Behind the scenes of seemingly nonstop campaigning, both candidates have been fighting a mixture of exhaustion and anticipation. Romney used his iPhone to take videos of sleeping staffers on his campaign plane. Obama brought aboard Air Force One some top aides from four years ago — Robert Gibbs, Reggie Love — for what seemed like a reunion of sorts on the final day of the campaign.

“You see the scars on me, the gray hairs on my hair to show I know how to fight for change,” Obama told a crowd in Wisconsin.

The president remains the slight favorite, with polls showing that he has a narrow lead in many of the swing states. He also has more pathways to get to the 270 Electoral College votes needed to win the election.

Of the 11 battleground states where recent polling averages show the candidates separated by fewer than 5 percentage points, Obama leads in nine.

After battling on the debate stage, and on the airwaves, the candidates on Tuesday largely turn their bids over to their ground game teams. Both campaigns have expansive operations in the swing states. The election could be decided by which party can best turn out their voters through bus rides, phone calls, and inspiration. It culminates months of work identifying possible supporters through sophisticated databases, then knocking on doors and making pitches.

In general terms, polls show Republicans more enthusiastic about the election and that could be reflected in the turnout. But snapshots from early voting states show massive turnout by Democrats in some urban precincts, a signal both of the voters’ level of support for Obama and of his highly-effective get-out-the-vote machine.

Attention once again started to turn to Florida and its election laws, with early voters having to wait as long as seven hours outside the polls. Governor Rick Scott, a Republican, refused to keep locations open longer. Even some Republicans criticized how the voting has been handled.

“I don’t know what went on in Florida, but I do have to say that in this day and age, it’s inexcusable that in this country, we have anything like this going on,” former New Jersey governor Christine Todd Whitman, a Republican, said on MSNBC.

The last day of early voting in Cleveland’s Cuyahoga County ended with Qiang Shen, 48, of Strongsville, a Chinese-food deliveryman and naturalized citizen who was casting his first-ever ballot for Obama.

“It’s a joy,” Shen said. “One vote may have no effect, but this experience is so exciting.”

After a series of courtroom battles this summer and fall put Ohio’s voting rules under a national microscope, the finale of early voting played out under an ashen sky. Despite the weather, the mood was festive; loudspeakers blared music from the steps of a nearby church, and a man in an Obama mask roamed the line, having his picture taken with voters.

Shen and his wife, Denise Luu, another first-time voter for Obama, had shown up in the parking area near the county’s election board as the seconds ticked by to the 2 p.m. close. “The guy told me, ‘you need to run,’ ” he said.

Some Cuyahoga County early voters reported lines of up to two hours Monday.

After rallies in Wisconsin and Ohio, Obama ended the night in Iowa, site of his surprise caucus win over favorite Hillary Clinton in 2008, a win that propelled him eventually to the presidency.

If he wins the states he campaigned in Monday, as well as those considered by most political analysts to be at least leaning toward Obama, he would earn the 270 Electoral College votes needed to win reelection.

Romney started the day in Florida, then headed to Virginia, Ohio, and New Hampshire. If Romney wins those four states, as well as those at least leaning toward him, he would become the nation’s 45th president.

Romney closed out his day in New Hampshire, the state where he launched his presidential campaign 524 days ago and where he delivered several of the major speeches of the campaign.

“This is a special moment for Ann and me, because this is where our campaign began,” Romney said, speaking to a crowd at the Verizon Wireless Arena in Manchester. “And then your primary vote put me on the path to win the Republican nomination. And tomorrow, your votes and your work right here in New Hampshire will help me become the next president of the United States.”

Alan Wirzbicki of the Globe staff and correspondent Callum Borchers contributed to this report. Matt Viser can be reached at maviser@globe.com.

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